In electrostatographic printing, commonly known as xerographic printing or copying, an important process step is known as “fusing.” In the fusing step of the xerographic process, dry marking material, such as toner, which has been placed in imagewise fashion on an imaging substrate, such as a sheet of paper, is subjected to heat and/or pressure in order to melt or otherwise fuse the toner permanently on the substrate. In this way, durable images are rendered on the substrates.
Currently, the most common design of a fusing apparatus as used in commercial printers includes two rolls, typically called a fuser roll and a pressure roll, forming a nip therebetween for the passage of the substrate therethrough. Typically, the fuser roll further includes, disposed on the interior thereof, one or more heating lamps, which radiate heat in response to a current being passed therethrough. The heat from the heating lamps passes through the surface of the fuser roll, which in turn contacts the side of the substrate having the image to be fused, so that a combination of heat and pressure successfully fuses the image.
In designing a fusing apparatus, there are a number of competing considerations. A thin-walled fuser roll is useful from the standpoint of rapid warm-up, but presents problems in distributing heat along the length thereof, such as causing “hot spots,” especially in areas where the print sheet does not contact the fuser roll to remove or absorb heat. Even when a feedback system is provided for controlling the temperature of the lamps within the fuser roll, the hot spot problem can persist.